I have the same problem. When I signed on to Steemit at the beginning of last September, I gathered all my keys, put in my posting key and never bothered it all this time. Never had any issue on anything using Steemit.
Last Saturday when I went to power up for the first time, of course I had to sign out so I could use my master key to enter the change. The power up worked fine. Later I logged out to put my posting key back in and after I signed out, it would not let me back in with the posting key nor master key. It did the same thing it is doing to you, it appears to sign in for about 2 seconds, then blinks back to the log in screen.
I got tired of messing with it and am using busy.
So you are not the only one and one of my friends says he saw someone else saying the same thing. Don't know how widespread that is.
Hi Jacey, thanks for letting us know that it is not an isolated issue. I have resolved it after a lot of faffing about with various key combos.
The big news is that the owner key and the memo key do not show on the steemit wallet when you login now (after you have signed up to the new wallet.steemit
Always backup your keys